In the background art, some light emitting devices of nitride semiconductor are configured to have a light emitting layer of nitride semiconductor that includes a multiple quantum well structure with quantum well layers and barrier layers each laminated alternately so that the main surface of the light emitting layer is a polar face; and current flows in a direction substantially vertical to the main surface.
The light emitting devices employ an InGaN well layer; and a GaN barrier layer or an InGaN barrier layer whose In-composition is lower than that of the InGaN well layer. The InGaN well layer is compressed because InGaN has a lattice constant larger than that of GaN.
A piezo electric field arises from compression strain. The piezo electric field separates holes and electrons both injected into the InGaN well layer spatially from each other to prevent radiative recombination.
An InGaN well layer with a thickness of 3 nm or less is often employed to prevent a reduction in the radiative recombination due to the piezo effect. Thinning the InGaN well layer causes holes and electrons to approach each other, thereby preventing the reduction in the radiative recombination of holes and electrons.
Unfortunately, driving a semiconductor light emitting device having an InGaN well layer that is 3 nm or less with a large current could cause excessively high carrier density in the InGaN well layer. As a result, an Auger recombination exceeds the radiative recombination. The Auger recombination increases with the cube of carrier density whereas the radiative recombination increases with the square thereof. Furthermore, carriers overflowing from the InGaN well layer increases. Thus, internal light emission efficiency decreases to cause a problem that semiconductor light emitting devices having a high optical output are not enabled.
In the background art, some light emitting devices of nitride semiconductor are configured to have a light emitting layer that includes a multiple quantum well structure with quantum well layers and barrier layers each laminated alternately so that current flows in a direction substantially vertical to the main surface of the light emitting device. The light emitting layer is provided between N-type and P-type semiconductor layers.
The semiconductor light emitting devices are each configured to have two or more barrier layers whose band gaps are equal to each other. Holes and electrons are injected into the semiconductor light emitting layers in the multiple quantum well structure from the sides of the P-type semiconductor layer and the N-type semiconductor layer, respectively.
Heavy holes stay mostly in the quantum well layers near the P-type semiconductor layer whereas light electrons reach the quantum well layers near the P-type semiconductor layer. As a result, the holes and the electrons are more likely to recombine with each other in the quantum well layers near the P-type semiconductor layer.
Holes and electrons could be confined to just one thin quantum layer to give rise to excessively high carrier density in some cases. This causes a problem that non-radiative Auger recombination proportional to the cube of the carrier density is more than radiative recombination proportional to the cube thereof, thus decreasing optical output.